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Glow Worm Glitter: New Research On Marine Luminescence

It's a lowly worm, the scale worm. Flat and mottled brown, the Harmothoe imbricata is nearly indistinguishable from its rocky surroundings on the intertidal zone.

Get it in the dark, however, and it's a different story. The scale worm is one of the sea's luminescent creatures. Scales on its back glow and even flash bright green. If the scales are pulled off, they glow where they fall.

Like lizards, scale worms can shed, then regenerate, parts of their bodies that have been destroyed by predators. Even the scales will regrow.

Are their flashes distress signals? Could their luminescence be related to courtship?

"See? There's a tiny baby there," says Perreault. She points proudly at a half-inch blip on a rock in the marine lab's saltwater tank. "It took me three days of rock flipping to find out how to look for them."

That may have been the easy part. Their scientific investigation has involved hours of time in a blackened research room lit only by infrared light. To test the worms' capacity for luminescence, they release the worm into a small container with a likely predator, such as a baby shore crab.

The worm monitoring lab, which operates in darkness, includes four video cameras which record worm-crab interactions. A photo multiplyer, which is part of the equipment, converts light to voltage to measure how bright the flashes of luminescence are.

With four video cameras recording the action, the researchers log information about their interplay. Data include the brightness and duration of the worm's glow, which scales illuminate, and, sadly, percentage of worms that are consumed by the crabs.

One of their most interesting observations appears to be a luminescent decoy maneuver on the part of the worm when its body is attacked or cut.

"The back half of the worm will glow, even if severed from the rest of the body," says Perreault. "The front half stays dark and moves away."

"The main theory that I wanted to test is that the back half is a successful decoy," says Rivers. "We do have videos that seem to support that. But watch this," he says. "Here's an additional defense behavior that is really spectacular to see."

Perreault keeps on eye on a crab before an experiment.

He replays a short video taken earlier in the week of a crab on top of a worm. All appears normal until two rows of neon-like lights fire brightly beneath the crab, which runs away.

Both laugh. "The crab appears to be startled away," observes Rivers. "It seems the worms may use lights to ward off predators before an attack by flashing, but once attacked they say, 'Okay, you take this decoy luminescence.' That allows the front half to quietly slip away to safety."

Rivers' research on scale worms is a continuation of earlier work he did on luminescence in marine animals. "I'm interested in visual signals from plankton to major predators," notes Rivers. "The ocean is so diverse and vision is extremely varied and fascinating to explore. It's really satisfying to be able to use research to tell a story."

Perreault's research is supported with a grant from the Henry L. and Grace Doherty Charitable Foundation Coastal Studies Research Fellowship.